They will stop releasing.. but even if they are discontinued it would take a very very much longer time than santa hat to rocket up. Santa had took about 5-6 years. This fish mask will properly take perhaps 10 years, 15 years, 20 years or maybe 35 years. Which is by then 99% of this generation of rs kid todays are married with a buncha kids and grandkids and had quited the game by then.
When santa hat/party hat was released, no one thought it would become such a rare and they were dropped all over. but this mask, are known to become rare by many people and people are already stocking them up. so even if it does become rare it will properly take a much longer time to increase its value to somewhere near the current santa
but who knows maybe one day jagex feel that fish mask should be worth more few years later.. and reedit fish mask with some bonus.. then it will really rocket up to santa hat real quick

They will be discontinued, but don't get your hopes up about the price rising anytime soon. A bunch of people without common marketing skills all think and probably still do think they'll rise substantially within just a year or two at the most, so they're stocking up on boat loads of spins to try and make a nice profit.
Since they're available for so long (this whole summer) Jagex will be claiming a bunch of cash from spins. I honestly think fish masks will cap @ 10m per mask after the summer, maybe around spring next year?

I dont think people realise the merchants with Billions will buy these items out due to low price now they've realised How much rare's can go to. Anyway fish masks are probably the same rarity as a santa and Partyhats due to the fact that Santas and P'hats/Crackers where dropped on a large scale, thats the reason why no one thought them of a rare. Fish masks will probably go for about 50m + the first day of verified discontinued, and keep rising. Then a dump will occur so price will drop (probably with bot nuke)